In recent years electronic equipment and apparatus have been improved so as to have a smaller size and a higher level of integration, and so there has arisen the very important task of removing the heat generated by various semiconductor devices including IC chips, mounted on electronic equipment and apparatuse. For the removal of the heat, various proposals have been made concerning part designing, circuit designing, materials and the like.
Currently, Al.sub.2 O.sub.3 is in use as a material for most of the substrates for semiconductors such as high integration IC and the like. However, with the recent improvement of ICs to a higher integration and a higher operating speed and the resulting increase in the amount of heat released by IC chips, there has arisen a demand for a material of higher heat releasability. Hence, BeO, SiC, etc. have been investigated as a substrate material. While both BeO and SiC have a high heat conductivity of about 260 W/mk, BeO is disadvantageous in that it is expensive and its dust has toxicity, and SiC is not easy to produce because it is not sufficiently sintered under normal pressure and so it must be sintered by hot pressing.
Hence, attention has been paid to aluminum nitride as a material for semiconductor substrates, having a higher strength than Al.sub.2 O.sub.3 and BeO, capable of being sintered at normal pressure and having a high heat conductivity. However, commercially available aluminum nitride powders usually contain about 2 to 3.5% by weight of oxygen and, from such a powder, it is difficult to produce an aluminum nitride sintered body of high heat conductivity, as described in Journal of the Ceramics Society of Japan, Vol. 93, No. 9, 1985, Pages 517-522 and in Electronic Ceramics, Vol. 16, No. 3, March 1985, pages 22-27. Hence, there was proposed a process comprising mixing appropriate proportions of alumina, ash and an alkaline earth metal, an yttrium alloy or the like in a liquid dispersing medium and then sintering the mixture in nitrogen or an ammonia atmosphere to obtain an aluminum nitride sintered body of increased purity and accordingly of improved sintering density and improved heat conductivity (Japanese Patent Laid-Open No. 60-65768).
There was also proposed an aluminum nitride sintered body obtained by adding to aluminum nitride for improving heat conductivity, boron nitride or an oxide of calcium, magnesium, aluminum, titanium, zirconium and/or a rare earth metal, preferably an yttrium oxide and then sintering the mixture (Japanese Patent Laid-Open No. 59-131583).
These aluminum nitride sintered bodies, however, have a large variation in heat conductivity between production lots and products of high heat conductivity cannot be obtained stably. These sintered bodies have a further problem that an aluminum nitride sintered body of high heat conductivity can be obtained only when the aluminum nitride used as a starting material contains a very low concentration of oxygen.
An aluminum sintered body can be used as a substrate for hybrid ICs whose patterning are not very fine, or as a package for high-integration logic circuit semiconductors. When the aluminum nitride sintered body is used as a substrate for hybrid ICs, even if a metallizing paste of a metal such as Mo, Mn or the like is directly printed on the surface of the sintered body and baked, the sintered body does not have a sufficiently high bonding strength to the metal. Therefore, it is desired that the bonding strength after baking be increased.
When the aluminum nitride sintered body is used as a substrate for semiconductor packages, ordinarily a silicon chip is mounted on the aluminum nitride substrate, and the upper surface of the silicon chip is covered by a ceramic such as aluminum nitride or the like with a lead frame connected to the silicon chip projecting outside through the interface between the aluminum nitride substrate and the ceramic cover. In order to increase the adhesion of the aluminum nitride substrate and the ceramic cover, the opposing surface portions of the aluminum nitride substrate and the ceramic cover which are in contact with each other are metallized and further the lead frame is soldered. However, aluminum nitride is difficult to metallize because it is chemically stable. Further, the aluminum nitride sintered body, having poor water resistance, tends to react with water to form ammonia, thereby being eroded.
In view of the above problems, it was attempted to laminate a metal such as silver, titanium, copper or the like on a substrate consisting of an aluminum nitride sintered body by means of vapor deposition to improve the wettability of the substrate. However, the metal layer thus formed by vapor deposition has a thickness of only about the order of .ANG. and it is very difficult to form the metal layer in a thickness of the order of .mu.m in order to secure a sufficient bonding strength to solder or a soldering material enabling the mounting of the devices. This is because when the metal layer is formed up to a thickness of the order of .mu.m, the internal stress becomes too large, making the metal layer highly likely to peel off.
Hence, an object of the present invention is to provide an aluminum nitride sintered body having a heat conductivity and a mechanical strength which are both sufficiently high for use in semiconductor substrates.
Another object of the present invention is to provide a process which can produce the above-mentioned aluminum nitride sintered body stably with substantially no variation in properties.
A further object of the present invention is to provide a highly heat-conductive semiconductor substrate obtained by subjecting the above-mentioned aluminum nitride sintered body to a surface treatment so that a paste of a metal such as Mo, Mn or the like can directily be printed on the resulting sintered body and baked to form a circuit pattern strongly adhering to the sintered body.
A still further object of the present invention is to provide a highly heat-conductive semiconductor substrate for packaging, having a strongly adhered metal layer formed by metallizing and so having improved wettability by solder or a soldering material.